Performance Analysis of Output Threshold-Based Incremental Multiple-Relay Combining Scheme with Adaptive Modulation for Cooperative Networks

Received Aug 23, 2017; Accepted Dec 12, 2017

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1. Introduction

In modern wireless communications, diversity techniques have received great attention to mitigate wireless channel impairments. Specifically, cooperative networks with the help of relays can be used to make broader coverage as well as to increase robustness against multipath fading [1]. However, cooperative networks lead to a loss in spectral efficiency because more resources (two frequency bands or two time slots) are used for half-duplex transmission [1]. To avoid such a loss in spectral efficiency by utilizing relays, incremental relaying scheme was proposed in [1] where a relay is used only if the direct transmission between the source and the destination does not support a predetermined performance. In other words, a relay is not used when the quality of the direct link exceeds a certain threshold, usually, in terms of signal-to-noise ratio (SNR).

There has been a lot of research studies on the cooperative network with multiple relays [2–7] because a spatial diversity gain can be achieved by utilizing multiple relays in a network like the antenna diversity system. However, utilizing multiple relays causes transmission efficiency losses as the number of relays increases. For examples, when m relays are utilized for the cooperative communication, it is required to have m+1 system resources. In order to avoid such huge losses, relay selection has been researched in the literature [2] that can obtain the diversity order as much as the space time-coded multiple-relay scheme [3] whereas the system complexity can be reduced by a use of only two resources. More specifically, opportunistic relaying was proposed in [2] where the system used only the best one among relays based on the relay selection criteria and proved that the same diversity-multiplexing tradeoff could be achieved as the space time-coded cooperative network [3]. Authors in [4] provided the diversity order of single relay...